Automotive ignition systems typically produce 20,000 to 40,000 volts at the ignition coil. The electrical current produced by this voltage is transferred from the distributor to the spark plugs of each cylinder via spark plug ignition cables.
A typical prior art spark plug ignition cable is shown in FIG. 1 and is denoted by the numeral 10. Prior art cable 10 comprises a conductor 12 surrounded by a primary insulation layer 14 made from silicone rubber or like material. The primary insulation layer 14 is surrounded by a braided cable reinforcing layer 16 made from stranded fiberglass. The braided reinforcing layer 16 is surrounded by a secondary insulating layer 18 of silicone rubber or like material which forms the outer jacket of the cable.
The braided reinforcing layer substantially prevents the cable from stretching when a tension force is applied to the cable. This prevents "thinning" of the cable, i.e., cable diameter reduction, and assures that metal terminals applied by crimping to each end of a completed spark plug cable assembly, do not slip off the cable when the cable is under tension.
The prior art cable 10 shown in FIG. 1 is manufactured according to the following prior art method which is described in conjunction with FIG. 3. In the first step of the method, the conductor 12 of cable 10 is fed into a first extruding machine 30 which extrudes the primary insulation layer 14 over the conductor 12. The cable 12 then passes through a first vulcanizing chamber 31 to vulcanize the primary insulation layer 14. As the cable emerges from the vulcanizing chamber 31, it is wrapped onto one of many spools 32 used for collecting the cable 10 at this stage of manufacture.
In the second step of manufacture, the primary insulation layer 14 is overbraided with the reinforcing layer 16 of stranded fiberglass. This is accomplished by a process which employs a multiplicity of braiding machines 34. Since, these braiding machines 34 operate at a much slower rate of speed as compared with the extrusion machines, many braiding machines are required to keep up with the production output of each extrusion machine.
In any event, the cable 10 collected on each spool 32 is fed into an associated braiding machine 34. Each braiding machine 34 overbraids the primary insulation layer 14 with the reinforcing layer 16 of stranded fiberglass. The cables 10 emerging from each of the braiding machines 34 are collected on another set of spools 36.
In the third step of manufacture, the cable 10 collected on each of the spools 36 is fed into a second extruding machine 38 which extrudes the secondary insulation layer 18 over the braided layer 16. The cable 10 then passes through a second vulcanizing chamber 39 to vulcanize the secondary insulation layer 18. After vulcanizing the secondary insulation layer 18, the cable 10 emerging from the vulcanizing chamber 39 is collected onto a third set of spools 40.
The overbraiding process describe above is very slow and costly. This is because each braiding machine 34 can only produce approximately ten feet of braided wire per minute. Thus, approximately thirty braiding machines are required to keep up with each extrusion machine which operates at approximately 300 feet per minute. Accordingly, substantial capital expenditures must be made to procure the required number of braiding machines. Each braiding machine requires routine maintenance, set-up time, and floor space which further increases the cost of manufacturing. Further, because multiple braiding machines are required per extruding machine, in-line processing is not possible. In particular, the cable coming out of the first vulcanizing chamber must be collected on multiple spools for processing on the braiding machines, instead of being fed directly into the next processing area.
It is, therefore, a primary object of the present invention to provide an improved spark plug ignition cable that can be manufactured more efficiently and at a lower cost than prior art spark plug ignition cables.